1. Field of the Invention
The present invention relates to a method of controlling operation of a refrigerator conduction cooling type superconducting coil and, more particularly, to a method of stably operating a refrigerator conduction cooling type superconducting coil constituting a superconducting magnet without quenching.
2. Description of the Background Art
Conventionally, a normal conductor such as copper and a metal based superconductor which is rendered superconductive at liquid helium temperature have been used for coils. When a high magnetic field is to be generated by using copper, it becomes necessary to cool by forced water flow, for example, as much heat is generated. A coil formed by using a normal conductor such as copper has the problems of large power consumption, difficulty in making a compact coil and laborious maintenance.
By contrast, a superconducting coil is useful in various applications, as a large magnetic field can be generated with a small power. However, when a metal based superconducting wire is used for a coil, cooling down to a cryogenic temperature (about 4 K) is necessary, resulting in much cost in cooling. Further, as the metal based superconductor is used at a cryogenic temperature with low specific heat, it is poor in stability and prone to quench.
Recently, techniques such as magnetic separation, crystal pulling and the like, which use an oxide high temperature superconducting coil which can be used at a relatively high temperature, have been proposed. The oxide high temperature superconducting coil can be used at a relatively high temperature as compared with the metal based superconducting coil, and therefore, can be used at a range with relatively high specific heat. It has been found that such use results in very good stability. Practical use of the oxide high temperature superconducting coil to make a more convenient magnet has been expected.
The oxide high temperature superconductor is rendered superconductive at liquid nitrogen temperature. At liquid nitrogen temperature, however, the oxide high temperature superconductor does not have very good critical current density and magnetic field characteristic at present. For this reason, the oxide high temperature superconductor has been used in a coil for generating a low magnetic field at present. The oxide high temperature superconducting coil, on the other hand, may possibly has higher performance at a temperature lower than liquid nitrogen temperature. For the use at a lower temperature, cooling by liquid helium is possible. The cost of cooling by liquid nitrogen, however, is high and the handling is difficult. In view of the foregoing, attempts using a refrigerator of which operation cost is relatively low and of which handling is simple have been made to cool the oxide superconducting coil to the cryogenic temperature.
The general method to find a stable operating range of the superconducting coil includes the steps of obtaining a load line and finding a stable operating range therefrom. An operating range derived from the load line is generally used for operating the metal based superconducting coils in both cases of a pool cooling type and a refrigerator conduction cooling type.
Similarly, the load line method may be used for an oxide high temperature superconducting coil. Here, the oxide high temperature superconductor has high critical temperature and makes a moderate transition to normal conduction, and therefore it has high stability and is not susceptible to quenching. It is expectable that, making use of this property, a current value in operating the coil can be increased to almost the critical current value. In addition, it is expectable that the operation current can be increased as much as possible while the oxide high temperature superconducting coil is cooled by a refrigerator of which operation cost is low and handling is easy. At present, however, on the oxide high temperature superconducting coil, its behavior in the refrigerator conduction cooling has not been sufficiently revealed, and therefore operation tests have to be done in order to find the stable operation range.